Cable television operators have typically been faced with telecommunications service solutions and architectures that were developed for other industries, classes of providers, scales, and physical plants. To date, two methods of providing voice services in the multimedia-rich cable industry have been proposed and are being tested: circuit switching and distributed telephony systems. Neither is well-suited to the need to carry a wide range of multimedia (video, audio, text, graphic, wideband, and narrowband) traffic over the limited geographic scale of the typical cable television outside plant, including but not limited to the types of hybrid fiber/coaxial cable (HFC) plants seen in the field today.
Circuit switching systems have been the standard switching means for primary voice quality and reliability in public telephony networks for many years. In such a system, circuit traffic is defined as having a pre-provisioned connection through a network. In particular, TDM-based circuit traffic is defined as having reserved bandwidth through the network, and more specifically, specific time slots through the network reserved to carry the traffic for that circuit whether or not any valid traffic is available to be sent. Certain standard TDM circuit formats have been defined such as DS0, DS1, and E1. Traditional methods for connecting TDM circuits together to complete a connection employ the use of a TDM-based switch. There are various architectures and ways to construct such a switch, but a general characteristic of such a switch is that once a connection is setup, there is no competition for switching resources, so a fixed latency through the switch is guaranteed.
In a distributed telephony system, such as that proposed by Cable Labs and others in the PacketCable™ initiative, telephony data is converted to packets and switched in a managed Internet Protocol (IP) environment, using a variety of IP and network protocols. The switch used in these types of systems, and for IP traffic in general, is typically referred to as a packet switch.
A packet switch is designed to handle packet traffic, which has different characteristics from circuit traffic. In particular, most packet systems are designed as connectionless, meaning they do not pre-provision a connection through the network, nor do they reserve bandwidth to carry the traffic. Some packet systems (for example, Asynchronous Transfer Mode [ATM] systems) do use connection-oriented protocols and some IP protocols (e.g., Multi-protocol Packet Label Switching [MPLS]) also provide a certain level of bandwidth reservation. However, these systems add extra complexity and potential compatibility issues.
In a packet switch, headers are attached to each individual packet to indicate the destination of the packet. The packets are switched in real-time to the correct output at each packet switch along the path. As a result, traffic arriving at a packet switch is non-deterministic and has to compete for switching resources as it tries to get through the switch. The resulting effect is that the packets are subject to a non-deterministic latency through the switch.
An additional characteristic of a packet switch is that it must be designed to handle different size packets. This is a result of the various protocols that are used in packet networks.
Typically, packets that are larger than the fixed size data unit (FSDU) are chopped into smaller pieces (i.e., fragmented or segmented). Packets that are smaller than the FSDU are padded to make a full FSDU. The size of the FSDU is arbitrary, although it is generally optimized to be efficient for the range of packet sizes expected in the application for which it is designed. An FSDU for a typical packet switch is between 64 bytes and 256 bytes.
As networks merge in the current telecommunication world, systems are being designed to accommodate both TDM circuit traffic and packet traffic simultaneously. Today the most common approach is to build a system that incorporates two separate switches, one that switches TDM data and one that switches packets.
This two switch approach arises from the above mentioned differences between circuit traffic and packet traffic. As discusses above, TDM-based circuit traffic has a reserved bandwidth through the network, and more specifically, specific time slots through the network reserved to carry the traffic for that circuit whether or not any valid traffic is available to be sent. There is no competition for switching resources, so a fixed latency through the switch is guaranteed. These switches can not handle packet traffic.
A packet switch does not pre-provision a connection through the network, nor do they reserve bandwidth to carry the traffic. Headers are attached to each individual packet to indicate the destination of the packet. The packets are switched in real-time to the correct output at each packet switch along the path. As a result, traffic arriving at a packet switch is non-deterministic and has to compete for switching resources as it tries to get through the switch. The resulting effect is that the packets are subject to a non-deterministic latency through the switch.
There remains a need in the art for an improved system that can switch both TDM traffic and packet traffic.